Various lighting technologies provide substitutes for sunlight in the 425-675 nm spectral region. In this spectral region, sunlight is most concentrated, and human eyes have evolved to be most sensitive. Technologies for efficiently creating visible light are continuously being developed. Such development may be viewed as the history of lighting.
A graph quantifying an aspect of the recent history of lighting is shown in FIG. 5. The vertical axis indicates luminous efficiency, in units of lumens per watt (“lumen” being a measure of light which factors in the human visual response to various wavelengths). The horizontal axis indicates time, in units of years A.D.
Three traditional lighting technologies are combustion, incandescence and high intensity discharges (HID). The progress of luminous efficiency of combustion, incandescence and HID technology are respectively represented by lines 30, 32, 34 in FIG. 5. The luminous efficacies of these technologies have made significant gains over the past 150 years. However, the progress appears to have virtually stalled in recent years. What is needed, therefore, is a lighting device with high luminous efficiency.